1. Field of the Invention
The present invention relates to a read head with a spin valve sensor that has a sense current employing sense current in plane (CIP) thence the sense current perpendicular to the plane (CPP) and, more particularly, to a first lead which is connected to an edge of the free layer of the spin valve sensor for conducting the sense current in plane in the free layer and a second lead layer, which may be a shield layer, which interfaces the film surface of the spin valve sensor for conducting the sense current perpendicular to the layers of the spin valve sensor, except for the free layer.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the sensor is at a minimum and when their magnetic moments are antiparallel the resistance of the sensor is at a maximum. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the sense current (IS) is conducted through the spin valve sensor, resistance changes detected by the sensor from the rotating magnetic disk cause potential changes that are detected and processed as playback signals. The sensitivity of the spin valve sensor is quantified as magneto-resistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magneto-resistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced.
The location of the transfer curve relative to the bias point is influenced by three major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demag field HD from the pinned layer and a sense current field HI from all conductive layers of the spin valve except the free layer. The strongest magnetic force on the free layer structure is the sense current field HI.
The problem of the sense current field on the free layer caused by conduction of the sense current IS through the other layers of the spin valve sensor can be overcome by conducting the sense current perpendicular to the film surfaces of the spin valve sensor (CPP) in contrast to conducting the sense current parallel to the film surfaces of the spin valve sensor (CIP). With this arrangement the first and second read gap layers may be eliminated with a bottom film of the spin valve sensor being electrically connected to the first shield layer and a top film of the spin valve sensor being electrically connected to the second shield layer. The first and second shield layers can serve the purpose of first and second lead layers for conducting the sense current perpendicular to the film surface planes of the spin valve sensor. In addition to eliminating the sense current fields on the free layer this arrangement has other advantages, namely: (1) a higher magnetoresistive coefficient dr/R than a typical spin valve sensor; (2) no shunting of the sense current past the free layer, which shunting results in the loss of the magnetoresistive coefficient dr/R; and (3) the thicknesses of the spacer and film layer structures becomes less critical which enables their thicknesses to be designed to counterbalance the aforementioned demagnetizing field and ferromagnetic coupling field on the free layer structure. A major problem with the sense current being conducted perpendicular to the film surface planes (CPP) of the spin valve sensor is that the spin valve sensor has insufficient resistance to the sense current to support the processing circuitry for detecting resistance changes to provide adequate playback signals. Accordingly, there is a strong-felt need to provide the CPP spin valve sensor with sufficient resistance so that the playback signals will be operable.
The present invention overcomes aforementioned problem of insufficient resistance of the spin valve sensor by conducting the sense current through the free layer structure parallel to its film surfaces thence perpendicular to the film surfaces of the other layers of the spin valve sensor. Each of the pinned layer structure, the pinning layer, the spacer layer, the free layer structure and the first and second shield layers have top and bottom film surfaces which are bounded by front and back edges and first and second side edges with the front edges being closer to the ABS than the back edges. First and second insulation layers are located between the first and second shield layers. The first lead layer is located between the first and second insulation layers and has top and bottom film surfaces which are bounded, in part, a front edge and first and second side edges. The front edge of the first lead layer abuts the back edge of the free layer structure for electrical connection thereto. One of the first and second shield layers serves as a second lead layer and has one of its first and second film surfaces electrically connected to one of the first and second film surfaces of the pinning layer.
In a preferred embodiment the pinned layer structure is an antiparallel (AP) pinned layer structure, which includes an antiparallel coupling layer located between first and second antiparallel (AP) layers. The antiparallel coupling layer is typically ruthenium (Ru) and is sufficiently thin so that the magnetic moments of the first and second AP pinned layers are antiparallel with respect to one another. With this arrangement the antiparallel pinned layer exerts a very low net demagnetizing field on the free layer structure. By appropriately sizing the thicknesses of the AP pinned layers and the spacer layer the ferromagnetic coupling field exerted on the free layer structure by the second AP pinned layer can be made to counterbalance the demagnetizing field from the AP pinned layer structure. Since there will be no sense current field exerted on the free layer structure biasing of the free layer structure for readback symmetry is more easily achieved.
In a preferred embodiment of the invention the first lead layer and the free layer structure are located closer to one of the shield layers so that a net image sense current field is exerted on the free layer structure parallel to its film surfaces and parallel to the ABS for stabilizing the magnetic domains of the free layer structure. It should be understood that in each of the shield layers there is an image current which is parallel to the sense current and is caused thereby. Therefore, in each shield layer there is an image current field which is caused by the image current in each shield layer. When the free layer is symmetrically placed between the first and second shield layers these image current fields completely counterbalance one another. In the present invention, by asymmetrically locating the free layer structure, the aforementioned image current field can be employed for magnetically stabilizing the free layer.
An object of the present invention is to provide a spin valve sensor wherein the sense current is conducted in such a manner through the spin valve sensor that the magnetoresistive coefficient dr/R is significantly increased over that of a spin valve sensor where the sense current is conducted parallel to the film surfaces of the spin valve sensor.
Another object is to provide a single spin valve sensor wherein the free layer is not subjected to a sense current field.
A further object is to provide the read head with a spin valve sensor wherein a sense current is conducted both perpendicular to and parallel to the film surfaces of the spin valve sensor with the free layer not being subjected to a sense current field but subjected to a net image current field for stabilizing its magnetic domains.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.